In a mass spectroscope, anion source generates ions, and causes the ions to travel in vacuum circumstance. The ions are introduced into a mass analyzer that separates the ions in accordance with the mass-to-charge ratio, by means of an electromagnetic force or a difference in time of flight, for example. In a case of a time-of-flight mass spectroscope that uses the above mentioned difference in flight time of ions, the ion source needs to give kinetic energy to ions in a pulsed manner (in a predetermined period of time). Thus it is important not only spatial convergence but also temporal convergence. Spatial convergence affects detection sensitivity, and temporal convergence affects mass resolution.
Ion sources used in time-of-flight mass spectroscopes have those which perform temporal convergence such as a two-stages acceleration method (see Non-Patent Document 1), a time lag focusing method, or an orthogonal acceleration method, and those which perform spatial convergence by forming a pinhole in a pull-out electrode (described later in detail). However, the ion sources used in time-of-flight mass spectroscopes do not include those which perform both temporal convergence and spatial convergence. A parallel and uniform field can be generated if a push-out electrode (described later in detail) and the pull-out electrode are infinitely large parallel plate electrodes. Therefore, in conventional cases, an ion source that supposedly performs both temporal convergence and spatial convergence is realized by using large parallel plate electrodes as the push-out electrode and the pull-out electrode, or increasing the size of the ion source.
Meanwhile, mass spectroscopes are widely used as indispensable analytical instruments in various fields such as analytical science, bioscience, pharmacy, medicine, environmental science, and space science. Previously, large-sized instruments installed in research institutes are normally used. In recent years, however, instruments were more and more often brought in on-site analysis. In this trend, there is an increasing demand for instrument portability. Therefore, the inventors have been making efforts to reduce the sizes of time-of-flight mass spectroscopes (see Non-Patent Documents 2 and 3), and there has been a need to form ion sources having smaller sizes than conventional sizes. As a result, it has become difficult to perform both temporal convergence and spatial convergence in an ion source. In the following, this aspect is described, with reference to FIGS. 5A and 5B.
FIGS. 5A and 5B are diagrams schematically showing the structure of an ion source 110 that is formed to have a smaller size than a conventional size. Reference symbol A in FIG. 5A and reference symbol B in FIG. 5B represent the trajectories of ions. The ion source 110 is an ion source using the above mentioned two-stages acceleration method.
The ion source 110 generally has three electrodes for outputting ions after generating the ions and causing the ions to converge, and an electron gun (not shown) that emits an electron beam. The three electrodes are, from the left-hand side of the drawing, a push-out electrode 101, a pull-out electrode 102, and a pull-in electrode 103. As shown in the drawing, the push-out electrode 101 is formed in a cup-like shape, so as to maintain the sealing properties of an ion generation area 104 and reduce the size of the ion source 110. The pull-out electrode 102 is designed to have a mesh-like form to allow ions to pass through. The pull-in electrode 103 has a hole through which ions are to pass. The electron beam emitted from the electron gun is surrounded by the push-out electrode 101 and the pull-out electrode 102, and is introduced into the ion generation area 101 in which ions are to be generated. To enable the introduction, a hole (not shown) through which the electron beam is to pass is formed in either side face of the push-out electrode 101 formed in a cup-like shape.
In the ion source 110, voltages of the same potential are applied to the push-out electrode 101 and the pull-out electrode 102, and the pull-in electrode 103 is set at 0 V (this is merely an example, and this potential is not necessarily used). With this arrangement, the push-out electrode 101 and the pull-out electrode 102 have a different potential from that of the pull-in electrode 103, and ions are generated in the ion generation area 104. After that, the pull-in electrode 103 remains at 0 V, voltages are applied so that a potential difference is created between each two of the push-out electrode 101, the pull-out electrode 102, and the pull-in electrode 103. The ions generated in the ion generation area 104 are pulled into an acceleration area existing between the pull-out electrode 102 and the pull-in electrode 103 by the electrical field formed by the push-out electrode 101 and the pull-out electrode 102. The ions pulled into the acceleration area are emitted through a hole in the pull-in electrode 103 while being accelerated. Since the push-out electrode 101 needs to output ions in a pulsed manner as described above, a pulse voltage is applied to the push-out voltage 101 at the time of ion acceleration.
As described above, in the ion source 110, ions that are accelerated simultaneously in two steps and have different initial locations in the ion source can be made to reach a specific location at the same time. Also, the specific location can be changed to any other location by appropriately adjusting the voltages to be applied to the push-out electrode 101 and the pull-out electrode 102.
An ion source 110 that is smaller than a conventional one, if conditions are set so that the isopotential lines that are the same as those of a conventional parallel plate electrodes are substantially parallel to one another as shown in FIG. 5A, the area from which ions can be pulled out becomes narrower and the amount of ions obtained becomes smaller, which resultes in a lower peak intensity. On the other hand, if conditions are set so that a large amount of ions can be pulled out as shown in FIG. 5B, the time convergence of ions becomes poorer due to the curves in the isopotential lines. Thus, the peak intensity and the peak width cannot have optimum values at the same time.
The above described energies are selected, because the time-of-flight mass spectroscope used by the inventors is of a multi-turn type that causes the ions output from the ion source to move around in the same space several times, so as to increase the distance of flight (see Non-Patent Documents 2 and 3), and a fan-shaped electrical field exists behind the ion source. If a electric sector field does not exist, it will be observed that both the peak intensity and the peak width become greater, though not shown in the drawings.